Optical switch having an autorestoration feature for switching from a backup optical path to a primary optical path

ABSTRACT

Optical switches are provided in an optical transmission system having at least two optical nodes in optical communication over a primary optical path and a backup optical path. An optical switch is located in each of the optical nodes. Each of the optical switches includes a switching element having an input port and a plurality of output ports coupled to the primary and backup optical paths, respectively. The switching element has a first state optically coupling an optical signal from the input port to the primary path and a second state optically coupling an optical signal from the input port to the backup path. First and second optical taps are located in the primary optical path. Third and fourth optical taps are located in the backup optical path. A first photodetector is optically coupled to the second optical tap for receiving a portion of the optical signal traveling in the primary optical path. A second photodetector is optically coupled to the third optical tap for receiving a portion of the optical signal traveling in the secondary optical path. A first optical path optically couples the first optical tap to the fourth optical tap such that a portion of an optical signal traveling in the secondary path is coupled onto the primary path. Finally, a controller is electrically coupled to each of the optical switches. The controller is configured so that when each of the switching elements are in the second state and the first photodetector in each of the optical switches detects an optical signal, the controller returns the switching elements to the first state.

FIELD OF THE INVENTION

[0001] The present invention relates generally to optical switches, andmore particularly to an optical switch that restores optical trafficfrom a secondary optical transmission path to a primary opticaltransmission path after a fault in the primary optical transmission pathhas been repaired.

BACKGROUND OF THE INVENTION

[0002] Currently, transmission systems employed in the cable television(CATV) industry provide two-way transmission of information; e.g.,video, audio, multimedia and/or data; between a head end and a pluralityof subscribers. Typically, the head end transmits the informationdestined for individual subscribers (“downstream information”) in anoptical format, via one or more fiber optic links, to one or moreoptical nodes. Each node converts the optically-formatted downstreaminformation into electrical signals for distribution, typically via acoaxial cable plant having a tree and branch architecture, to individualsubscribers. In addition to receiving the downstream information, eachindividual subscriber may generate information in the form of voice,video, data, or any combination thereof, destined for the bead end. Thesubscriber-generated information (“upstream information”) is aggregatedby the coaxial cable plant and passes to the node for conversion into anoptical format for transmission to the head end.

[0003] CATV service providers and their subscribers are accustomed tohigh reliability service. One way in which high reliability is achievedis by providing two optical paths between the head end and each opticalnode, one of which serves as a primary optical path and the other ofwhich serves as a secondary or backup optical path. An optical switchswitches the optical information signals from the primary path to thesecondary path in the event of an unanticipated failure in the primarypath. The optical switches are often located in the head end and theoptical nodes.

[0004] The aforementioned optical switches generally employ anoptomechanical switching component that switches between the primarypath and the secondary path based on the electrical voltage that isapplied to it. A portion of the optical signal in the primary andsecondary paths is tapped off and converted to an electrical voltage.The voltages are monitored and if a threshold condition is violated,indicating a failure in the primary path, the switch is activated sothat traffic is transferred to the secondary path. Unfortunately, theoptical switch does not include any arrangement for switching back fromthe secondary to the primary path after the primary path has beenrestored. Rather, an operator or technician must perform a manual powercycle to restart the optical switches in both the head end and theoptical node so that the switches return to the primary path.Restoration in this manner can be difficult because the head end and theoptical node may be located 50 to 100 km apart from one another. Also,there may be many such optical switches in both the head end and thenodes, thus requiring the operator to take proper care to ensure thatthe correct combination of switches are power cycled so that there is nointerference with traffic on the other paths.

[0005] Accordingly, it would be desirable to provide a method andapparatus for automatically restoring optical traffic from a secondaryoptical transmission path to a primary optical transmission path after afault in the primary optical transmission path has been repaired withoutthe need to perform a manual power cycle.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, optical switches areprovided in an optical transmission system having at least two opticalnodes in optical communication over a primary optical path and a backupoptical path. An optical switch is located in each of the optical nodes.Each of the optical switches includes a switching element having aninput port and a plurality of output ports coupled to the primary andbackup optical paths, respectively. The switching element has a firststate optically coupling an optical signal from the input port to theprimary path and a second state optically coupling an optical signalfrom the input port to the backup path. First and second optical tapsare located in the primary optical path. Third and fourth optical tapsare located in the backup optical path. A first photodetector isoptically coupled to the second optical tap for receiving a portion ofthe optical signal traveling in the primary optical path. A secondphotodetector is optically coupled to the third optical tap forreceiving a portion of the optical signal traveling in the secondaryoptical path. A first optical path optically couples the first opticaltap to the fourth optical tap such that a portion of an optical signaltraveling in the secondary path is coupled onto the primary path.Finally, a controller is electrically coupled to each of the opticalswitches. The controller is configured so that when each of theswitching elements are in the second state and the first photodetectorin each of the optical switches detects an optical signal, thecontroller returns the switching elements to the first state.

[0007] In accordance with one aspect of the invention, the two opticalnodes respectively comprise a head end and an optical node in a CATVtransmission system.

[0008] In accordance with another aspect of the invention, the switchingelement is an optomechanical switching element.

[0009] In accordance with yet another aspect of the invention, the firstand second photodetectors are photodiodes.

[0010] In accordance with another aspect of the invention, the primaryand secondary optical paths are unidirectional paths. Alternatively, theprimary and secondary optical paths may be bi-directional paths.

[0011] In accordance with another aspect of the invention, a method isprovided for switching optical traffic from a secondary optical path toa primary optical path, each of which establish an optical communicationpath between first and second optical nodes. The method begins by:detecting a presence or absence of an optical signal traveling in theprimary path through the first optical node; detecting a presence orabsence of an optical signal traveling in the primary path through thesecond optical node; detecting a presence or absence of an opticalsignal traveling in the secondary path through the first optical node;and detecting a presence or absence of an optical signal traveling inthe secondary path through the second optical node. A portion of anoptical signal traveling in the first optical node is coupled from thesecondary path to the primary path. A switching element is switched ineach of the first and second optical nodes from a second state to afirst state so that the optical traffic traverses the primary opticalpath when an optical signal is detected traveling in the primary paththrough both the first and second optical nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows a simplified block diagram of a conventionalarrangement for providing a primary and second optical path between thehead end and an optical node in a CATV transmission system.

[0013]FIG. 2 shows a simplified block diagram of an arrangement forproviding a primary and second optical path between the head end and anoptical node in a CATV transmission system in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014]FIG. 1 shows a simplified block diagram of a conventionalarrangement for providing a primary and second optical path between thehead end and an optical node in a CATV transmission system. Opticalswitches 110 and 120 are located in the head end and the optical node,respectively. Optical transmission path 112 serves as the primary pathwhile optical transmission path 114 serves as the secondary or backupoptical path. Traffic along the primary path 112 and the secondary path114 may be unidirectional or bi-directional. Optical switch 110 includesan optomechanical switching element 116, tap couplers 117 and 118 andphotodiode 121 and 122. Likewise, optical switch 120 includes anoptomechanical switching element 124, tap couplers 125 and 126 andphotodiodes 127 and 128. A switch controller 127 controls the operationof switches 110 and 120.

[0015] Referring to switch 110, tap couplers 118 and 121 respectivelycouple a small portion of the optical signals traveling in paths 112 and114 to photodiodes 121 and 122. Controller 123 receives the electricalsignals from the photodiodes 121 and 122 and determines the position ofthe optomechanical switching element 124. As shown, switch 120 isconfigured in a manner similar to switch 110.

[0016] In operation, switches 110 and 120 are initially in states A andB, respectively. That is, the switches 110 and 120 provide a continuousoptical path to points A and B on the primary path so that the signalsare transmitted along the primary path 112. If transmission along theprimary path 112 is now lost because of a fiber break, the controller127 will respectively force the optomechanical switching elements 116and 124 to switch from positions A and B to positions A′ and B′,respectively. As a result, traffic is now transported along thesecondary path 114. As previously mentioned, transmission will continuealong the secondary path 114 even after the primary path 112 has beenrestored. The only way to restore switches 110 and 120 to states A andB, respectively, is to perform a manual power cycling in which theoptomechanical switching elements 116 and 124 return to their initialstates. This limitation is overcome with the inventive optical switchesdepicted in FIG. 2.

[0017]FIG. 2 shows a simplified block diagram of an arrangement forproviding a primary and second optical path between the head end and anoptical node in a CATV transmission system in accordance with thepresent invention. Optical switches 210 and 220, which are located inthe head end and the optical node, respectively, switch optical trafficbetween primary transmission path 212 and secondary transmission path214. Optical switch 210 includes an optomechanical switching element216, tap couplers 217, 218, 230 and 232 and photodetectors 221 and 222.Likewise, optical switch 220 includes an optomechanical switchingelement 224, tap couplers 225, 226, 240 and 242 and photodetectors 227and 228. A switch controller 227 controls the operation of switches 210and 220. Each switch 210 and 220 has its own controller because theswitches are often located 50-100 km apart.

[0018] Optomechanical switching elements 216 may be any arrangement thatemploys physical motion of one or more optical elements to performoptical switching. In this way, a spatial displacement of a reflectedbeam is affected. Photodetectors 221, 222, 227 and 228 may be anycomponent that converts an optical signal received from the tap couplersto an electrical signal such as a photodiode, for example.

[0019] Referring to switch 210 in more detail, tap couplers 218 and 221respectively couple a small portion of the optical signals traveling inpaths 212 and 214 to photodiodes 221 and 222. In addition, tap couplers230 and 232 are also located in the primary path 212 and the secondarypath 214, respectively. Tap coupler 230 couples a small portion of theoptical traffic traveling along the primary path 212 and directs italong optical fiber 234 to tap coupler 232. Tap coupler 232, in turn,couples the portion of the optical traffic received from primary path212 onto the secondary path 214. That is, a portion of the traffictraveling along the primary path 212 is placed on the secondary path214. Likewise, tap coupler 232 couples a small portion of the opticaltraffic traveling along the secondary path 214 and directs it alongoptical fiber 234 to tap coupler 230. Tap coupler 230, in turn, couplesthe portion of the optical traffic from the secondary path 214 onto theprimary path 212. That is, a portion of the traffic traveling along thesecondary path 214 is placed on the primary path 212. As shown, switch220 is configured in a manner similar to switch 210.

[0020] In operation, assume switches 210 and 220 are in states A′ andB′, respectively, as a result of a fiber break along the primary path212. A small portion of the optical signal traveling in the secondarypath 214 is coupled to the primary path 212. Photodetector 222 in switch210 detects the signal but, because of the fiber break, photodetector228 in switch 220 will not detect the portion of the signal tapped fromthe secondary transmission path 214. However, when the primary path 212has been restored, both photodetectors 222 and 228 will detect theportion of the signal tapped from the secondary transmission path 214.In response to the signals detected by both photodetectors 222 and 228,controller 227 activates the optomechanical switching elements 216 and224 so that the switches 210 and 220 are returned to state A and B. Thatis, the transmission is automatically restored to the primary state.Power cycling is not required at either the head end or the opticalnode.

[0021] The state of the optomechanical switching elements 216 and 224 isdetermined by the voltage that is applied to them via electrical boardsincorporated into the switches 216 and 224. A threshold condition isestablished for the switches 216 and 224 in software that determine thevalue of the voltage pulse (or current pulse) that changes their state.The threshold condition can be adjusted either by the operator or can befactory-set based on the distance over which the signal is transmittedand customer requirements.

[0022] The analog voltages generated by the photodetectors are directedto the controller 227 via logarithmic amplifiers, which are used byfirmware to determine the appropriate state of the optomechanicalswitching elements. The firmware then sends a voltage pulse (or currentpulse) to the optomechanical switching elements 216 and 224 to switchthem from the primary path to the secondary path, or visa versa.

[0023] Although various embodiments are specifically illustrated anddescribed herein, it will be appreciated that modifications andvariations of the present invention are covered by the above teachingsand are within the purview of the appended claims without departing fromthe spirit and intended scope of the invention. For example, while theinvention has been described in terms of an optical switch that providesa secondary or backup path in a CATV system, the optical switch moregenerally may be employed in any optical transmission system in which abackup path is to be provided.

1. In an optical transmission system having at least two optical nodesin optical communication over a primary optical path and a backupoptical path, an optical switch located in each of the optical nodes,each of said optical switches comprising: a switching element having aninput port and a plurality of output ports coupled to the primary andbackup optical paths, respectively, said switching element having afirst state optically coupling an optical signal from the input port tothe primary path and a second state optically coupling an optical signalfrom the input port to the backup path; first and second optical tapslocated in the primary optical path; third and fourth optical tapslocated in the backup optical path; a first photodetector opticallycoupled to the second optical tap for receiving a portion of the opticalsignal traveling in the primary optical path; a second photodetectoroptically coupled to the third optical tap for receiving a portion ofthe optical signal traveling in the secondary optical path; and a firstoptical path optically coupling the first optical tap to the fourthoptical tap such that a portion of an optical signal traveling in thesecondary path is coupled onto the primary path; and a controllerelectrically coupled to each of the optical switches, said controllerbeing configured so that when each of the switching elements are in saidsecond state and the first photodetector in each of the optical switchesdetects an optical signal, said controller returns the switchingelements to said first state.
 2. In the system of claim 1 wherein saidtwo optical nodes comprise a head end and an optical node in a CATVtransmission system.
 3. In the system of claim 1 an optical switchwherein said switching element is an optomechanical switching element.4. In the system of claim 1, an optical switch wherein said first andsecond photodetectors are photodiodes.
 5. In the system of claim 1, anoptical switch wherein the primary and secondary optical paths areunidirectional paths.
 6. In the system of claim 1, an optical switchwherein the primary and secondary optical paths are bi-directionalpaths.
 7. A method for switching optical traffic from a secondaryoptical path to a primary optical path, each of which establish anoptical communication path between first and second optical nodes, saidmethod comprising the steps of: detecting a presence or absence of anoptical signal traveling in the primary path through the first opticalnode; detecting a presence or absence of an optical signal traveling inthe primary path through the second optical node; detecting a presenceor absence of an optical signal traveling in the secondary path throughthe first optical node; detecting a presence or absence of an opticalsignal traveling in the secondary path through the second optical node;coupling a portion of an optical signal traveling in the first opticalnode from the secondary path to the primary path; and switching aswitching element in each of the first and second optical nodes from asecond state to a first state so that the optical traffic traverses theprimary optical path when an optical signal is detected traveling in theprimary path through both the first and second optical nodes.
 8. Themethod of claim 7 wherein said two optical nodes comprise a head end andan optical node in a CATV transmission system.
 9. The method of claim 7wherein said switching element is an optomechanical switching element.10. The method of claim 7, wherein the detecting steps are performed byphotodiodes.
 11. The method of claim 7, wherein the primary andsecondary optical paths are unidirectional paths.
 12. The method ofclaim 7, wherein the primary and secondary optical paths arebi-directional paths.